The loss has Atletico's top four hopes at risk.
Koke said: "Until the goal we were good, with the second it already hurt us and cost us. Then Barcelona is a very complicated opponent, it is a tough and difficult night.
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"It's reality, we all have to be aware of what we're at stake, tighten our a**** and look forward, because we have to be fourth.
"The first half was competitive, the key was the second goal, a hard blow. In the second half it was more difficult for us.
"More than concern, we have to be on alert because if we compete like we did against Inter, like we did in the Cup and Champions League or like we do at home, we can achieve the club's goal and what we all want to be among the top four."